1. Field of the Invention:
This invention relates to a distance measuring device arranged to measure a distance to an object by transmitting a signal such as a light to the object and by receiving reflection signals from the object and particularly to a distance measuring device of the light projecting type operating on the principle of triangulation or trigonometrical measurement and having computative processing means such as an electrical circuitry arranged to adequately process analog signals coming from a photometric circuit for distance measurement and to produce a continuous distance detecting signal.
2. Description of the Prior Art:
There have been known distance measuring device of the kind called the triangulation distance measuring type having light beams projecting means and light sensitive elements arranged in a row to receive a light reflected from a distance measuring object and arranged to detect which of the light sensitive elements has received the reflection light by means of a photometric circuit corresponding to these light sensitive elements. Generally, in a distance measuring device of this type, each light sensitive element corresponds to one distance zone. The light receiving signal of each light sensitive element is reduced to presence or absence of the light receiving signal according to a predetermined threshold valve so that a distance zone in which the distance measuring object is located can be determined. For example, in the case of a device represented by FIG. 1 of the accompanying drawings, the device comprises a light source 1 which is a light emitting diode (LED) or the like, a lens 2 for projecting a light beam, an array 3 of three light sensitive elements arranged in a row and a light receiving lens 4. A reference numeral 5 in FIG. 1 indicates an object the distance to which is to be measured. In this device, the three light sensitive elements give three measurable distance zones. In a distance measuring device of this type, the reflection light coming from an object located within the farthest distance zone is extremely weak. In view of this, there has been proposed a method in which such a distance that gives no light receiving signal that reaches the threshold value at any of the light sensitive elements 3a, 3b and 3c is regarded as the farthest distance zone. In accordance with this method, the device shown in FIG. 1 can be arranged to be capable of discriminating four distance zones.
Meanwhile, a Japanese Patent Application Laid-Open No. Sho 56-26212 has disclosed a method. According to this method, in a distance measuring device of the type mentioned above which is arranged to have the light receiving signal of each light sensitive element converted into a binary value representing the presence or absence of the light receiving signal according to the above stated threshold value, if more than two adjoining light sensitive elements detect a reflection light from a distance measuring object, the middle part of the distance zones corresponding to the light receiving signals thus obtained is regarded as a distance zone in which the object is located. According to this method, therefore, it is possible to set additional distance zones in the middle parts between the adjoining distance zones. Then, the above stated distance measuring device of FIG. 1, for example, can be operated as having five distance zones.
In any case, however, in accordance with the conventionally known methods, the number of distinguishable distance zones cannot be increased by more than two times. In cases where precise and elaborate distance measurement with an increased number of distinguishable distance zones is required, the conventional methods necessitates an increase in the number of light sensitive elements. However, in order to increase the number of the light sensitive elements within a distance measuring device in an optical arrangement, the light receiving area of each light sensitive element must be reduced. Then, this results in a decrease in quantity of light incident upon each light sensitive element. This means degradation of the noise resistivity of the distance measuring device. Further, the width of light intensity distribution of light incident upon the array of light sensitive elements is limited by the defocus and aberration of the light beam projecting and light receiving optical systems. Therefore, even when the resolution of the light sensitive element array is enhanced by the increased number of light sensitive elements, the number of distinguishable distance zones does not substantially increase.